In recent years, a positioning function using a GPS (Global Positioning System) is incorporated into various electronic devices such as a car navigation device, a mobile phone, and a digital still camera. Typically, when the GPS is used in the electronic device, a GPS module receives a signal from four or more GPS satellites and measures a position of the device based on the received signal, and the measurement result is notified to the user via, for example, a screen of a display device. To be more specific, the GPS module demodulates the received signal to acquire orbital data of each GPS satellite, and obtains the device's three-dimensional position by solving simultaneous equations with the orbital data, time information, and a delay time of the received signal. The reason for requiring the four or more GPS satellites, from which the signal is received, is to remove influence of an error between the time inside the module and the time of the satellite.
Here, the signal (the L1 band, a C/A code) transmitted from the GPS satellite results from data of 50 bps being subjected to spread spectrum with a Gold code of a code length of 1023 and a chip rate of 1.023 MHz, and the spread spectrum signal further being subjected to BPSK (Binary Phase Shift Keying) modulation using a carrier of 1575.42 MHz. Therefore, to receive the above signal from the GPS satellite, the GPS module needs to synchronize the spread code, the carrier, and the data.
Typically, the GPS module incorporated into the electronic device converts a carrier frequency of the received signal into an intermediate frequency (IF) of within some MHz, and then carries out the synchronization processing described above. A typical intermediate frequency is, for example, 4.092 MHz, 1.023 MHz, and 0 Hz. Usually, the signal strength of the received signal is smaller than that of a thermal noise, and the S/N is lower than 0 dB. However, the signal can be demodulated by a processing gain of a spread spectrum method. In the case of a GPS signal, the processing gain with respect to the data length of 1 bit is 10 Log (1.023 MHz/50)≈43 dB, for example.
For many years, the GPS receivers had been mainly used for car navigation systems. However, in recent years, the GPS receivers have been incorporated into digital still cameras (hereinafter, DSC) and the like, and the market of the GPS receivers tends to expand. In terms of performance, sensitivity has been improved, and the GPS receivers having receiver sensitivity of −150 to −160 dBm have been popular. This is contributed by the fact that a large-scale circuit can be produced at low cost because of enhancement of the degree of integration of an IC by miniaturization of semiconductor processing. Power consumption has been also lowered.
In the use of the GPS receiver for a typical car navigation system, basically, continuous positioning (typically, once every second) is performed. Since the power is supplied from a large battery of the car, the power consumption during operation has not often become a problem. Meanwhile, a simplified navigation system (personal navigation device—PND) of recent years, a mobile phone, a DSC, and other mobile devices have a small battery, and those except the PND do not necessarily require the continuous positioning. For the mobile devices, battery survival time is a very important element, and the case must be avoided in which the battery survival time is extremely shortened because the incorporated GPS receiver is operated as before, so that the primary function of the mobile devices is impaired. As described above, reduction of the power consumption of the GPS receiver of recent years has been enhanced. However, the power consumption during the continuous operation is not sufficient for the mobile devices. Therefore, there are many cases in which the mobile devices operate with lower power by intermittent operation. Frequency of positioning can be reduced by the intermittent operation, and when the positioning is not performed, by decreasing most of the power except the power of a part of the circuit or of the circuit as a whole, an effect of reliably reducing the average power can be expected.
The intermittent operation in the GPS receiver is to cause the GPS receiver to be in a sleep condition when the positioning is not performed, in which the operation other than that of a minimum necessary circuit stops, so that an hourly average of the power is decreased and the power is lowered. The minimum necessary circuit that operates during the sleep condition is typically a real time clock having a low frequency (hereinafter, RTC, the frequency is typically 32.768 kHz) and a back-up memory for storing the satellite's orbit, time information, and the like. To function the intermittent operation, it is necessary to reestablish synchronization of the received signal from each satellite in a short time after returning from the sleep condition.
The simplest way to reestablish the synchronization of the received signal is to carry out, after returning from the sleep condition, an initial set-up that is the same as a normal set-up when the power of the GPS receiver is turned ON. The GPS receiver's normal initial set-up is divided into three types: cold start, warm start, and hot start, depending on whether or not ephemeris and almanac which are orbital information of the satellite are available. The ephemeris is orbital information that is individually transmitted from the satellite and has a short term of validity although it is accurate enough to be used for a positioning calculation. The almanac, on the other hand, is rough orbital information that is commonly transmitted from all of the satellites, has a long term of validity, and is useful to specify an available satellite from which a signal is received. The cold start is used for an initial set-up when neither orbital information is available, the warm start is used for an initial set-up when only the almanac is available, and the hot start is used for an initial set-up when both of the orbital information are available. The former two require about 30 seconds before a start of positioning, whereas the hot start requires few seconds, and even 1 second or less is possible under preferable conditions.
In the method of intermittent operation in which a normal initial set-up of a GPS receiver is carried out for reestablishing the synchronization, it is typical to transfer to the intermittent operation after establishing initial positioning by the cold start or the warm start, and then to carry out the hot start by which the positioning can be performed in a short time. In this method, a synchronization acquisition unit for acquiring the synchronization with respect to the received signal from the satellite operates in the GPS receiver. Since the synchronization acquisition unit has a large processing load, and there are many cases of consuming much larger power than a synchronization holding unit which holds the synchronization, it is inconvenient when a battery is defined by the peak power rather than the average power.
To reduce the peak power, there is a way to reestablish the synchronization using only the synchronization holding unit rather than using the synchronization acquisition unit. In order to realize this, it is necessary to have a method for maintaining highly accurate time information in the sleep period, and for restarting a synchronization holding circuit with accuracy of within 1 chip (1/1.023μ seconds) of a spreading code after returning from the sleep condition. When there is the accuracy of within 1 chip of the spreading code, a delay-locked loop (DLL) for carrying out the synchronization of the spreading code can synchronize the spread code instantly. Typically, the synchronization holding unit has a plurality of synchronization holding circuits for holding the synchronization, simultaneously receives signals from a plurality of satellites, and holds the synchronization with the respective satellites. However, because of accuracy and stability of an oscillation frequency of an oscillator which measures time, the longer the sleep period is, the more difficult it becomes to maintain the highly accurate time information.
To maintain the highly accurate time information during the sleep period, there is a way to store a result of a low accurate RTC frequency (several tens of ppm) measured by a counter of a highly accurate GPS reception oscillator (it is typical to use a temperature-compensated TCXO, and 0.5 ppm is an example for GPS) before the sleep, and, after returning from the sleep condition, to correct an error to the elapsed time by the RTC using the measured result of before the sleep (Patent Literature 1). By using this method, only the RTC operates during the sleep while the GPS reception oscillator is stopped, and the synchronization can be reestablished after returning from the sleep without using the synchronization acquisition unit, whereby considerable reduction of the power can be expected.